Rapid Thermal Processing (RTP) is a versatile optical heating method which can be used for semiconductor processing as well as a general, well controlled, method for heating objects or wafers which are in the form of thin sheets, slabs, or disks. The objects are generally inserted one at a time into a chamber which has at least some portions of the chamber walls transparent to transmit radiation from powerful heating lamps. The transparent portion of the walls is generally quartz, which will transmit radiation up to a wavelength of 3 to 4 microns. These lamps are generally tungsten-halogen lamps, but arc lamps or any other source of visible and/or near infrared radiation may be used. The radiation from the lamps is directed through the transparent portions of the walls on to the flat surface of the object to be heated. Radiation may be directed on to the flat surface of the object from one side or the other, or both sides simultaneously. As long as the objects absorb light in the near infrared or visible spectral region transmitted by the transparent portion of the walls, RTP techniques allow fast changes in the temperature and process gas for the different material processes and conditions. Since the flat surface of a semiconductor wafer may be uniformly irradiated, the entire wafer may be heated with relatively little temperature difference across the wafer during the entire time of heating, and hence little slip occurs. RTP allows the "thermal budgets" of the various semiconductor processing to be reduced, as well as allows the production of various metastable states which can be "frozen in" when the material is cooled rapidly.
RTP systems are relatively new. In the last 10 or 15 years, such systems were used only in research and development. The thrust of the work was increasing the temperature uniformity, and developing heating cycles and processes which decreased the thermal budget. Prior art RTP machines can heat unstructured, homogeneous materials in the form of a flat plate or disk, and produce temperature uniformities across the plate adequate for semiconductor processing processes.
The temperature control in current RTP systems is mostly performed by monochromatic (or narrow wavelength band) pyrometry measuring temperature of the relatively unstructured and featureless backside of semiconductor wafers. The results of the temperature measurement are generally used in a feedback control to control the heating lamp power. Backside coated wafers with varying emissivity can not be used in this way, however, and the backside layers are normally etched away or the temperature is measured using contact thermocouples.
A newer method of temperature control is the power controlled open loop heating described in U.S. Pat. No. 5,359,693, which patent is hereby incorporated by reference.
German patent DE42 23 133 C2, hereby incorporated by reference, discloses a method of producing relatively defect free material in RTP machines. Apparatus induced thermal inhomogeneities have been reduced in the last few years because of the demand for more uniform processing. Among the techniques used have been control of the individual lamp power, use of circular lamps, and rotation of the semiconductor wafers with independent power control.
Most RTP machines have a thin rectangular quartz reaction chamber having one end open. Chambers meant for vacuum use often have a flattened oval cross section. Chambers could even be made in the form of a flat cylindrical pancake. In general, the chambers are used so that the thin objects to be heated are held horizontally, but they could also be held vertical or in any convenient orientation. The reactor chamber is usually thin to bring the lamps close to the object to be heated. The reactor chamber is opened and closed at one end with a pneumatically operated door when the wafer handling system is in operation. The door is usually made of stainless steel, and may have a quartz plate attached to the inside. The process gas is introduced into the chamber on the side opposite the door and exhausted on the door side. The process gas flow is controlled by computer controlled valves connected to various manifolds in a manner well known in the art.
In the march of semiconductor technology to ever smaller device dimensions and ever larger wafer sizes, the depths of implant of dopant atoms is getting more and more shallow, and the allowed tolerances for implant dose and movement of these dopant atoms is getting tighter and tighter. Rapid thermal processing has allowed engineers to keep to the tighter tolerances since the processing can be done at the optimum temperature for the particular process, while spending very little time at other temperatures on the way up and on the way down in the temperature vs time curve.
We have found, however, a previously unrecognized problem in the processing of silicon devices having bare silicon surfaces, or only a native oxide or with very thin gate and tunnel oxides. The processing itself seems to change the distribution and the amount of dopant in some cases. We have found that previous process gas compositions sometimes lead to such problems, and that proper specification of the process gas composition for each particular type of wafer, temperature, and time combination, can result in much improved uniformity of the devices produced using the RTP process.